Electrophotographic marking is a well-known method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes black and white electrophotographic marking. Electrophotographic marking can also produce color images by repeating the above process for each color of toner that is used to make the composite color image. For example, in one color process, referred to as the REaD 101 process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptor is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. A recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are then placed in superimposed registration so that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
One way of exposing a photoreceptor is to use an LED (light emitting diode) printbar-based exposure station. Such exposure stations are generally comprised of an elongated array of LEDs and an array of gradient index lenses that focus the light from the LEDs onto the photoreceptor. One goal of an LED print-bar based exposure station is the production of compact irradiance distributions on the photoreceptor. Deviating from compact distributions tends to increase bluriness and noise in the resultant printed image. FIG. 1 illustrates the spatial relationship between a light emitting diode 10 of an LED printbar, lens elements 12 of a gradient-index lens array, and a light spot 14 produced on a photoreceptor 15. To achieve high resolution (usually measured in spots per inch, or SPI) an LED printbar will typically have a large number of individual LEDs. Each LED images a small section, referred to as a pixel, of the latent image. By selectively driving the individual LEDs according to input video data a desired latent line is exposed. By moving the photoreceptor as lines are exposed a two-dimensional latent image results.
As shown in FIG. 1, the gradient index lens array is positioned between the light emitting diodes of the LED array and the photoreceptor. Gradient index lens arrays, such as those produced under the trade name "SELFOC" (a registered trademark in Japan that is owned by Nippon Sheet Glass Company, Ltd.) are comprising of bundled gradient index optical fibers, or rods, reference U.S. Pat. No. 3,658,407. That patent describes a light conducting rod made of glass or synthetic resin which has a cross-sectional refractive index distribution that varies parabolically outward from the center of the rod. Each rod acts as a focusing lens for light introduced at one end. Relevant optical characteristics of gradient index lens arrays are described in an article entitled "Optical properties of GRIN fiber lens arrays: dependence on fiber length", by William Lama, Applied Optics, Aug. 1, 1982, Vol. 21, No. 15, pages 2739-2746. That article is hereby incorporated by reference.
Ideally, light from a light emitting diode produces a narrow, well-defined latent image on the photoreceptor. This requires that the photoreceptor be exposed with a narrow light spot having sufficient power to fully expose the photoreceptor. A measure of the width of the light spot is the full width half maximum (FWHM) distance, the distance between the light spot's half power points. FIG. 2 illustrates various irradiance profiles from the light emitting diode 10 of FIG. 1. Assuming that the light emitting diode 10 has an exemplary active area geometry 16, the light emitting diode emits light with a radiance distribution profile 18. That light passes through the gradient index lens elements 12, which impart a spreading function 20 to the light. The result is an irradiance profile 22 that can be characterized by a FWHM distance 24, the distance between the half power points.
While LED printbar based exposure stations are generally successful, they have problems. One problem relates to degradations in irradiance profiles caused by light emitting diodes having less than ideal active area geometries. FIG. 3 illustrates the irradiance profiles from a light emitting diode having an active area geometry 26 that is less than ideal because an electrode 36 divides the active area into two sections The light emitting diode then emits light with a radiance distribution profile 28 that is distorted. That light passes through a gradient index lens array, which again imparts a spreading function 20 to the light. The result is an irradiance profile 30 having a FWHM distance 32 that is significantly greater than the FWHM distance 24 of FIG. 2.
The result of the greater FWHM distance is a wider irradiance profile than is desired. Therefore, LED printbars having light emitting diodes with geometries that produce a more compact radiance profile would be beneficial. Even more beneficial would be electrophotographic marking machines that use LED printbars having light emitting diodes with a geometry that produces a more compact radiance profile.